1. Field of the Invention
Embodiments of the invention generally relate to a method and apparatus for detecting wafer slipouts from a polishing head.
2. Background of the Invention
In semiconductor wafer processing, the use of chemical mechanical planarization, or CMP, has gained favor due to the enhanced ability to increase device density on a semiconductor workpiece, or substrate, such as a wafer. Chemical mechanical planarization systems typically include a polishing head and a platen that supports a polishing material. The polishing head generally includes a ring that circumscribes a substrate receiving pocket in which the substrate is retained during processing. Processing of the substrate is generally performed by providing relative motion between the substrate and the polishing material in the presence of a polishing fluid while pressing the substrate against the polishing material.
During polishing, frictional forces between the substrate and the polishing material causes the substrate to be forced laterally against the ring of the polishing head. Occasionally, a triggering event causes the retainment of the substrate within the polishing head to become partially or completely lost. For example, some of the pressure biasing the ring towards the polishing material may be lost, thus diminishing the force capturing the substrate between the polishing head and the polishing material. If the pressure is sufficiently reduced, the lateral force of the substrate against the ring may cause the ring to lift thus allowing the substrate to escape from under the polishing head. Other triggering events may include passing the substrate over a polishing surface abnormality such as wrinkles in the polishing material and run-out in the parallelism between the polishing head and platen.
Once the wafer has slipped out from under the polishing head, the substrate may be scratched or broken. Additionally, if the slipout event is not timely detected, valuable production time is lost while the damaged wafer waiting to be removed from the polisher. Additionally, the non-retained wafer left in the processing area may damage the tool or tool components such as sensors or wiring.
Therefore, there is a need for a method and apparatus for detecting wafer slipouts.
In one aspect of the invention, an apparatus for detecting disengagement of a workpiece is provided. In one embodiment, the apparatus includes a polishing head and a detector. The polishing head has a first portion and a second portion. The detector is adapted to provide a metric indicative of relative motion between the first portion and the second portion.
In another aspect of the invention, a method for detecting disengagement of a workpiece from a polishing head is provided. In one embodiment, the method includes the steps of pressing the workpiece retained in the polishing head against a polishing material, providing relative motion between the workpiece and the polishing material, and detecting motion of the polishing head in a direction normal to the polishing material.